It's tempting to agree with Bob Kagan--that the 21st century will indeed be just like the 19th. Yet there's (at least) one crucial difference: Following J.A. Hobson's analysis, 19th century imperialism was largely driven by a quest for new markets, while 21st century "imperialism" is mostly driven by a quest for security of supply, in order to up the living standards of 7 billion capitalists. In other words, the 19th century was a supply-driven, demand-side century. But the 21st century is a demand-driven, supply-side century.
This should theoretically make the coming century less violent: Competition for resources may become intense, but it will likely remained confined to countries that have those resources--rather than ranging across every square-inch of the globe, as it did in the era of imperialism. Conflicts about oil, for example, will play out in Sudan and Iraq--unlike the 19th century conflicts that resulted in colonial wars everywhere subjects could be found, from Cape Town to Russia's Far East.
Furthermore, implicit in supply-side competition is the idea that, to "win," all you need to do is diversify. That means there's a huge incentive to develop cheap, alternative sources of energy and synthetic building materials--something that governments from China to Arizona are attempting feverishly to do. This is an organic "way out," built in to the structure of this century's great game.
Of course, Hobson's theory of imperialism can't explain everything--no era is truly "all demand" or "all supply," and security drives politics as much as economics. Moreover, spiking nationalism may yet drive countries to compete when it is not wise (especially in places like Taiwan). But times have changed, and the underlying structures are different. Despite superficial similarities, the 21st century is not the 19th--and to say it is so might distort the truth.--Barron YoungSmith